One type of semiconductor package is referred to as a "chip scale package". Chip scale packages are also referred to as "chip size packages", and the dice are referred to as being "minimally packaged". Chip scale packages can be fabricated in "uncased" or "cased" configurations. Uncased chip scale packages have a "footprint" (peripheral outline) that is about the same as an unpackaged die. Cased chip scale packages have a peripheral outline that is slightly larger that an unpackaged die. For example, a footprint for a typical cased chip scale package can be about 1.2 times the size of the die contained within the package.
Typically, a chip scale package includes a substrate bonded to the face of the die. The substrate includes the external contacts for making outside electrical connections to the chip scale package. The substrate for a chip scale package can comprise a flexible material, such as a polymer tape, or a rigid material, such as silicon, ceramic, glass or FR-4. The external contacts for one type of chip scale package include solder balls arranged in a dense array, such as a ball grid array (BGA), or a fine ball grid array (FBGA). These dense arrays permit a high input/output capability for the chip scale package. For example, a FBGA on a chip scale package can include several hundred solder balls.
One aspect of chip scale packages is that the dense arrays of external contacts are difficult to fabricate. In particular, reliable electrical interconnections must be made between the external contacts for the package, and contacts on the die contained within the package. Typically, the contacts on the die are thin film aluminum bond pads in electrical communication with integrated circuits on the die.
FIG. 1 illustrates a prior art chip scale package 10. The package 10 includes: a semiconductor die 12; a polymer tape 14 bonded to a face of the die 12; and an encapsulant 16 bonded to the face and sides of the die 12. In addition, the package 10 includes an adhesive layer 18 for bonding the polymer tape 14 to the die 12, and a dense array of solder balls 20 formed on the polymer tape 14. Metal beams 22 are bonded to the solder balls 20, and to device bond pads 24 on the die 12. The metal beams 22 are also encapsulated in the encapsulant 16.
A representative process flow for forming the chip scale package 10 includes bonding one or more dice 10 to a strip of the polymer tape 14. The metal beams 22 can then be bonded to the device bond pads 24. Next, the encapsulant 16 can be formed, and the solder balls 20 attached to the metal beams 22. The individual packages 10 can then be singulated from the strip of polymer tape 14 and tested.
Typically, a thermosonic bonding process using gold or gold plated materials is employed to bond the metal beams 22. In addition, specialized bonding tools are required to make the bonds between the metal beams 22 and the bond pads 24. The metal beams 22 are also subjected to stresses from the bonding and encapsulation processes, and during subsequent use of the package 10. These stresses can cause the bonds to weaken or pull apart.
The present invention is directed to an improved chip scale semiconductor package including dense array external contacts, and improved interconnects between the external contacts and contacts on the die.